That would make sense. The other locos are heavy enough to provide necessary braking power, while the lighter WAP5 would always need the added precaution.
But it's surprising that they are letting such long trains descend on a 1:37 gradient without brakers! That's rather close to the line!
The reason for switching to P7 for SKD and NGP Duronto were the compressor constraints in the conventional locos I suppose.